The present invention generally relates to an apparatus for measuring the thickness of a paint coating on a substrate such as a metallic sheet or the like, and more particularly to an apparatus for continuously measuring the thickness of a paint coating by using infrared rays.
Conventionally, color painted metallic sheets and the like have been widely employed in various industrial fields for producing products such as building materials, appliances, food cans, etc. In the case of painting on such metallic sheets, it is important to achieve a desired precise thickness of the paint coating, so that a measuring apparatus with high accuracy has been required for this purpose.
Up to the present, there have been provided measuring apparatuses utilizing radiation, for example, of X rays or .beta. rays. However, these apparatuses have such disadvantages that owing to the strict legal requirements imposed on them, they cannot be readily operated or utilized by unqualified persons, while the measuring accuracy obtainable thereby is not necessarily so high, and therefore, these measuring apparatuses have been extremely limited in their applications.
Recently, there has been provided a measuring apparatus utilizing infrared rays instead of X rays and .beta. rays, for example in the U.S. Pat. No. 3,973,122. This type of measuring apparatus utilizes the characteristic properties such that some part of the infrared rays including a specific wave length can be absorbed by the paint coating comprising high polymer when the rays pass through the paint coating.
The measuring apparatus shown in said U.S. patent comprises a black-body radiator for emitting beams of infrared rays of a predetermined wave length to a film laminated on a substrate, and an electronic pyrometer for receiving the beams of the infrared rays reflected from the surface of the substrate and measuring the thickness of the paint coating on the basis of the intensity of the reflected infrared rays, some part of which has been absorbed by the paint coating in the passage therethrough.
However, the conventional measuring apparatus as described above still has a disadvantage in that a high measuring accuracy cannot be constantly obtained with respect to various kinds of paint coatings.